Magnetic developer for electrostatic photography and process for preparation thereof

Abstract

A developer for electrostatic photography comprising as indispensable components a fine powder of a magnetic material and a binder resin, said developer consisting essentially of a dry blend of (A) substantially spherical particles of a composition comprising a medium of said binder resin and said fine powder dispersed in the binder resin medium and (B) a finely divided solid conducting agent having a particle size smaller than 1/10 of the size of the spherical particles (A), said spherical particles (A) having a crater-like rough surface, said finely divided solid conducting agent (B) being stuck to said spherical particles (A) through said crater-like rough surfaces thereof, and the volume resistivity of said dry blend being not higher than 1 × 10 11 Ω-cm.

What we claim is: 
     
       1. A developer for electrostatic photography consisting essentially of a dry blend of (A) substantially spherical particles of a composition comprising a binder resin and a fine powder of a magnetic material dispersed in the binder resin so that the magnetic material is distributed predominantly in the surface layer of the spherical particles to form a crater-like rough surface; and   (B) a finely divided solid conducting agent comprising electrically conductive carbon black and having a particle size smaller than 1/10 of the size of the spherical particles (A), the solid conducting agent being stuck to spherical particles (A) through the crater-like rough surfaces thereof without being embedded by the dry blending of spherical particles (A) and solid conducting agent (B), and being present in an amount of up to 2% by weight, based on spherical particles (A), the dry blend having a volume resistivity of not higher than 1 × 10 11  Ω-cm.   
     
     
       2. The developer as set forth in claim 1 wherein the fine powder of the magnetic material is composed of triiron tetroxide having an average particle size not larger than 500 millimicrons. 
     
     
       3. The developer of claim 2 wherein the fine powder of the magnetic material is selected from the group consisting of triiron tetroxide, diiron trioxide, zinc iron oxide, ytterium iron oxide, cadmium iron oxide, copper iron oxide, lead iron oxide, nickel iron oxide, neodium iron oxide, barium iron oxide, magnesium iron oxide, manganese iron oxide, lanthanum iron oxide, iron powder, cobalt powder, and nickel powder. 
     
     
       4. The developer as set forth in claim 1 wherein the binder resin medium is a synthetic resin which shows adhesiveness under the application of heat or pressure. 
     
     
       5. The developer as set forth in claim 1 wherein the synthetic resin is an epoxy resin. 
     
     
       6. The developer as set forth in claim 1 wherein the fine powder of the magnetic material is present in the spherical particles (A) in an amount of 20 to 80% by weight, based on the spherical particles (A). 
     
     
       7. The developer as set forth in claim 6 wherein the fine powder of the magnetic material is present in spherical particles (A) in an amount of 40 to 60% by weight, based on the spherical particles (A). 
     
     
       8. The developer as set forth in claim 1 wherein the spherical particles (A) have an oil absorption of 45 to 90. 
     
     
       9. The developer as set forth in claim 1 wherein the spherical particles (A) are those obtained by mixing the fine powder of the magnetic material with a dispersion or solution formed by dispersing or dissolving the binder resin in a liquid mixture of a water-miscible organic solvent and a water-immiscible organic solvent, and mixing the resulting mixture with an aqueous medium under high shearing agitation sufficient to cause granulation of the mixture, whereby the organic solvents in the resulting particles of said mixture are transferred into the aqueous medium. 
     
     
       10. The developer as set forth in claim 1 wherein the solid conducting agent is dry-blended in the spherical particles (A) in an amount of 0.1 to 2% by weight, based on the spherical particles (A). 
     
     
       11. The developer as set forth in claim 1 wherein spherical particles (A) additionally contain dyes, pigments and extender pigments having a particle size smaller than that of the fine powder of the magnetic material and present in an amount less than 50% by weight, based on the final composition. 
     
     
       12. The developer as set forth in claim 11 wherein the dyes, pigments and extender pigments are present in an amount less than 10% by weight, based on the final composition. 
     
     
       13. A process for the preparation of developers for electrostatic photography comprising: (I) mixing a powdery magnetic material with a binder resin dissolved or dispersed in a liquid mixture of: (a) a water-miscible organic solvent, and   (b) a water-immiscible organic solvent, the weight ratio of (a) to (b) ranging from 10:1 to 1:10;     (II) mixing the resulting mixture of (I) with an aqueous medium under strong shearing agitation sufficient to cause granulation of the mixture, thus transferring the organic solvents into the aqueous medium to form substantially spherical particles wherein the magnetic material is distributed predominantly in the surface layer portion of the spherical particles to form a crater-like rough surface;   (III) recovering the so formed particles;   (IV) water-washing the particles of (III);   (v) drying the particles of (IV) under such conditions that the resin binder is not substantially melted; and   (VI) dry-blending into the particles of (V) a finely divided solid conducting agent having substantially a particle size of less than 1/10 the size of the particles of (V), the conducting agent being stuck to the particles of (V) through the crater-like rough surfaces thereof.   
     
     
       14. The process according to claim 13 wherein the resin concentration in the solution or dispersion of the binder resin in the liquid mixture of the water-miscible organic solvent and the water-immiscible solvent is 5 to 40% by weight. 
     
     
       15. The process of claim 14 wherein the resin concentration in the solution or dispersion of the binder resin in the liquid mixture of the water-miscible organic solvent and the water-immiscible solvent is 10 to 20% by weight. 
     
     
       16. The process of claim 13 wherein the binder resin is a synthetic resin which shows adhesiveness under the application of heat or pressure. 
     
     
       17. The process of claim 16 wherein the synthetic resin is an epoxy resin. 
     
     
       18. The process of claim 13 wherein the solid conducting agent is carbon black, aluminum powder, copper powder, or silver powder. 
     
     
       19. The process of claim 13 wherein the solid conducting agent is present in an amount of up to 2% by weight, based on the spherical particles. 
     
     
       20. The process of claim 13 wherein the solid conducting agent is present in an amount of 0.1 to 2% by weight, based on the spherical particles. 
     
     
       21. The process of claim 13 wherein the powdery magnetic material has a particle size of less than 500 mμ and is selected from the group consisting of triiron tetroxide, diiron trioxide, zinc iron oxide, ytterium iron oxide, cadmium iron oxide, copper iron oxide, lead iron oxide, nickel iron oxide, neodium iron oxide, barium iron oxide, magnesium iron oxide, lanthanum iron oxide, manganese iron oxide, iron powder, cobalt powder, and nickel powder.